Although the invention in this disclosure is described with reference to copolymerization of propylene and ethylene, especially sticky polymers, it will be understood that the invention can be readily applied to the polymerization or copolymerization of other alpha-olefin monomer combinations such as ethylene-hexene, polyethylene, polypropylene, propylene-butene, propylene-hexene and also terpolymer systems.
Sticky polymers are defined as polymers which, although particulate at temperatures below the sticking or softening temperature, agglomerate at temperatures above the sticking or softening temperature. A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into agglomerates of much larger sized than the original particles. To avoid agglomeration problems in the past, these polymers have been produced at temperatures below the softening temperature. However, U.S. Pat. No. 4,994,534 has taught a process for polymerizing sticky polymers at temperatures at or above the softening point of the sticky polymers in the presence of an inert particulate material (carbon black, silica, clays, talc, and other like materials). This patented process generally enhances the yield of product in relation to the catalyst and makes purging of the polymer product more efficient. In this polymerization process, sticky polymers are produced in a single reactor or by two or more reactors in series. Typically, the polymerization reaction in a gas phase reactor is catalyzed by a transition metal catalyst.
Unfortunately, during normal operations, the inner surfaces of the recycle gas lines and the inner surfaces of the tubes of the heat exchanger or cooler tend to foul with undesirable polymer deposits. Fouling in the recycle gas lines reduces gas flow and impedes the heat exchanger's capability of cooling recycled gas. As a consequence, the reactor must be shut down within a short time for cleaning. There is a need to reduce polymer build-up in the recycle lines and heat exchangers during the polymerization of sticky polymers in gas phase fluidized reactor system. U.S. Pat. No. 5,037,805 has disclosed a method for inhibiting polymer buildup in a heat exchanger during gas phase polymerization of alphaoolefins by introducing upstream of the heat exchanger para-ethyl-ethoxybenzoate (PEEB) in an amount sufficient to inhibit polymer buildup. However, there is an on-going need for additional methods for inhibiting polymer buildup in recycle lines and heat exchangers employed in gas phase fluidized bed polymerization processes.